One of the biggest tech wonk debates in HD voice, SIP and transitioning to an all-IP network has been over the necessity for a master database to associate IP addresses with phone numbers. I'm nearly convinced that we'll need some formal registry solution in the future.
The no-database side argues an eNum style directory writ large is unnecessary. "Let SIP be SIP and let the end-points figure out the connection," they argue. "Phone numbers are obsolete and the IP communications system of the future doesn't need legacy 10 digit numbers or the overhead to dip into a database to figure out if a device can accept an HD voice or video call. Everything will be figured out during the SIP call setup."
In truth, there is merit to this approach and that's how a lot of SIP sessions are making HD voice and video calls today. If you are calling Joe via your SIP-enabled video device, you and Joe have already exchanged the basic idea that Joe also has a SIP-enabled video device since he sent you a URL to use to make the call.
Pro-directory people argue that directory lookup doesn't add a lot of latency to call setup -- after all, it's been happening in the legacy PSTN to provide caller ID information for decades without appreciable delays. Look up would also provide call efficiency since the dialing SIP client would be informed in advance about the capabilities of the other device, thereby avoiding some excess handshaking to sort out codecs and video support.
Where the directory headache comes in is registering and updating devices. Presumably this happens automatically or with minimal human intervention, but a phone number has to be linked with an IP address at some point. Phone number linkage may also translate into geographic linkage to a particular address for E-911 with broadband service.
But this headache is also an asset when it comes to authenticating user identity on the network -- a point which the no-directory crowd doesn't really address. In today's Wild West "We'll just dial" VoIP world, there's an infinite pool of phone numbers that are anonymous and/or can be caller ID spoofed. Regulators around the world are concerned that bad actors can all too easily leverage today's loosey-goosey VoIP/PSTN hybrid world for everything from annoying spam calls to outright fraud.
A master trusted directory (Or directories, to be redundant and not to provide a monopoly to any one company or source) could be linked into a PKI infrastructure to provide secure authentication of a record. If a phone number doesn't map to an expected IP address after a lookup, the call can be flagged and/or simply declined.
Some will argue there are other alternatives to a phone number-style database registry, but I've been swayed that a directory is necessary for tighter security to cut down on fraud, malicious caller ID spoofing such as "SWAT" dialing, and just to cut down on the amount of randomly unsolicited calls people get every day -- even with a national no-call directory.